INNER STRUCTURE OF THE KRAKATAU VOLCANIC COMPLEX (INDONESIA) FROM GRAVITY AND BATHYMETRY DATA

On 27 August 1883, the Krakatau volcanic complex (Indonesia) was the s ite of one of the most destructive historical eruptions. Most of the v olcano was destroyed and a new caldera also formed during this catastr ophic event. Since the date of the eruption, many geological studies o f the superficial structures and eruption products have been carried o ut. A debate on the scenario of the eruption and the way the volcano c ollapsed has developed and still is unresolved. In order to assess the inner structure of the volcanic complex, we carried out a detailed la nd and marine geophysical survey in the summer 1990. In this paper, ba thymetry and gravity data collected during the survey as well as liter ature data are compiled and analysed. Bathymetric data show that the c aldera is characterized by a flat sea-bottom at 240 m below sea level and by steep linear walls suggesting that the caldera collapse has bee n controlled by pre-existing features. Moreover, the build-up of the y oung active volcano, Anak Krakatau, on the very edge of the caldera co uld lead to mechanically unstable conditions which must be considered for hazard mitigation. The Bouguer anomaly of the volcanic complex is characteristic of volcanoes with an explosive behaviour. 3-D gravity m odelling reveals the previously unknown geometry of the dense substrat um of the proto-Krakatau and evidences the presence of a collapsed str ucture beneath the caldera filled up with low-density material. Finall y, we point out a major weakness zone, oriented N150 degrees on a line passing through the old and recent vents of the volcano. This zone co uld have guided both the development of the volcanic activity and the emplacement of the 1883 caldera. Furthermore, this weakness zone passi ng through the summit line of the pre-1883 Krakatau volcano has been i ntroduced as a significant disruption surface of the volcanic edifice in the updated scenario of the 1883 eruption that we propose.